We propose to definitively test the hypothesis that a prominent mechanism of action of adrenocorticotropic hormone (ACTH) is the alteration of specific ion permeability of the cell membrane resulting in transmembrane ionic currents. Our previous findings indicate that ACTH does influence general membrane electrophysiological properties. We now propose to use a single-electrode voltage-clamp device to define general and specific ion current flow under resting and ACTH-stimulated conditions in the superfused mouse and rat adrenal cortex. The biological relevance of these ion-currents from a population of cells will be ascertained by relating them to steroid production by the same tissue. Steady state and total steroidogenic response to secretagogues will be determined by a specific RIA for corticosterone. Our studies are specifically designed to: 1. Characterize total resting ion currents for up to 8 hrs in young adult mouse zona fasticulata cells. This will include defining the longitudinal stability of the total ion current from a single cell and a population of cells for the 8 hrs. We will examine whether single cell ion events accurately predict changes in resting activity of the population. 2. Examine the current-voltage relationship which exist in resting adrenocortical cells for up to 8 hrs, in order to initially evaluate the nature of the resting currents. 3. Describe the nature of the general ionic currents which occur following ACTH administration. 4. Define the nature of the component ion currents following ACTH stimulation by removing specific ionic constituents from the superfusion medium and chemically blocking specific ion current flow and relating this information to the steroidogenic response of the tissue under these conditions. 5. Determining the specificity and generality of the ion current responses by employing inactive analogues, structurally unrelated secretagogues and other target cells of ACTH. As a result of these studies, we will be able to obtain definitive data to support the hypothesis that a general mechanism whereby peptide hormones regulate target cell function is the alteration in membrane permeability to specific ions. The affirmation of this hypothesis and the clear description of specific ion permeability changes will permit a more rational search for pharmacological approaches to the modification of physiological and pathological cellular processes regulated by peptide hormones.